Crustal structure

Granite-greenstone belts display a variety of structural styles and outcrop patterns, many of which also occur in Phanerozoic orogens (Kusky & Vearncombe, 1997). Those common to both Archean and Phanerozoic belts include large tracts of metamorphosed igneous and

4 km

20 km

28 km

Oceanic crust section

Hydrothermally altered

Gabbro

Ultramafic cumulates

rvi^rA

^---TiJ

K^Granulite/ 1 eclogite

Delamination:

Melts of pyroxenites

are basalt/nephelinite

4 km

13 km 18 km

Oceanic crust section Hydrothermally altered

Gabbro

Ultramafic cumulates

-------- ^tm~

\ \ Subduction:

Melts of garnet

^ amphibolite are TTG

Basalt

Fig. 11.7 Two-stage model of Archean crustal evolution (after Foley et al., 2003, with permission from Nature 421, 249-52. Copyright © 2003 Macmillan Publishers Ltd). (a) Early Archean delamination of thick oceanic crust and the melting of pyroxenites (1). Local melting of lower crust (2) and garnet amphibolite (3) may also occur to produce small volumes of felsic magma. (b) Late Archean whole-crust subduction and the large-scale melting of garnet amphibolite to produce TTG suites (3).

sedimentary rock that are deformed by thrust faults and strike-slip shear zones (e.g. Figs 10.13, 10.19). Another pattern, commonly referred to as a dome-and-keel architecture, occurs exclusively in Archean crust. This latter structural style forms the focus of the discussion in this section.

Dome-and-keel provinces consist of trough-shaped or synclinal keels composed of greenstone that surround ellipsoidal and ovoid-shaped domes composed of gneiss, granitoid, and migmatite (Section 9.8). The contacts between domes and keels commonly are high-grade ductile shear zones. Marshak et al. (1997) distinguished between two types of these provinces. One type has keels composed of greenstones and their associated metasedimentary strata (Section 11.3.2) and domes composed of granitoid rock that is similar in age or slightly younger than the greenstones. The other type has domes of mostly gneissic and migmatitic basement rock and keels composed of greenstones that are younger than the dome rocks.

The Eastern Pilbara craton of northwestern Australia (Fig. 11.8) is one of the oldest and best preserved examples of a granite-greenstone belt and dome-and-keel province with a history spanning 3.722.83 Ga (Collins et al., 1998; Van Kranendonk et al., 2002, 2007). The craton exposes nine granitoid domes with diameters ranging from 35 km to 120 km. Studies of seismic refraction data and gravity and magnetic anomalies (Wellman, 2000) indicate that the margins of the domes are generally steep and extend to mid-crustal depths of ~14 km. Despite their simple outlines, the internal structure of the domes is complex. Each contains remnants of 3.50-3.43 Ga TTG suite granitoids (Section 11.3.2) that are intruded by younger (3.33-2.83 Ga) more potassic igneous suites (Fig. 11.9a). The domes display compositional zona-tions and variable degrees of deformation. In many cases, the youngest bodies are located in the cores of the domes with older, more deformed granitoids at the margins. This internal structure indicates that each dome was constructed through the emplacement of many intrusions over hundreds of millions of years and that deformation accompanied the magmatism.

Between the granitoid domes are synclinal tracts of greenstone composed of dipping volcanic and sedimentary sequences up to 23 km thick (Van Kranendonk

Fig. 11.8 Geologic map of the Pilbara granite-greenstone belt (modified from Zegers & van Keken, 2001, with permission from the Geological Society of America, with additional structural information from Van Kranendonk et al., 2007) showing the typical ovoid pattern of TTG suite granitoids surrounded by greenstones belts. ME, Mt. Edgar Dome; CD, Corunna Downs Dome; S, Shaw Dome. Black box shows location of Fig. 11.9.

Fig. 11.8 Geologic map of the Pilbara granite-greenstone belt (modified from Zegers & van Keken, 2001, with permission from the Geological Society of America, with additional structural information from Van Kranendonk et al., 2007) showing the typical ovoid pattern of TTG suite granitoids surrounded by greenstones belts. ME, Mt. Edgar Dome; CD, Corunna Downs Dome; S, Shaw Dome. Black box shows location of Fig. 11.9.

Greenstones

3 Gorge Creek Group clastic sedimentary rock Wyman Formation (c.3325-3315 Ma) rhyolite Euro Basalt

□ Panorama Formation (3458-3426 Ma) Basalt-dacite, minor rhyolite

Apex Basalt

Yj Duffer Formation (3471-3463 Ma) Basalt-dacite

Talga Talga Subgroup (c.3490-3477 Ma) Basalt, minor felsic volcanic rock, chert

Granitic rocks

^ Split Rock Supersuite c.2830 Ma High-K monzogranite JU Cleland Supersuite c.3240 Ma High-K monzogranite

Emu Supersuite (younger) c.3295 Ma High-K monzogranite T] Emu Supersuite (older) c.3325-3308 Ma High-K monzogranite Tambina Supersuite c.3450-3430 Ma TTG Quartz porphyry c.3467 Ma ■ Callina Supersuite c.3470 Ma TTG

Mt. Edgar Shear Zone

C Greenstone

Mt. Edgar Shear Zone

Granitic complex

Mt. Edgar

Shear Zone Greenstone

Mt. Edgar Shear Zone

Granitic complex

Mt. Edgar

Shear Zone Greenstone

Greenstone

Granitic complex Greenstone

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How To Have A Perfect Boating Experience

How To Have A Perfect Boating Experience

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